Turning is a ubiquitous task for human ambulation, and this task has been shown to be related to falling and fall-related injuries in Parkinson's Disease patients and for the elderly, and likely creates difficulties for lower limb amputees as well. Unlike an intact leg, which transfers loads to the skeletal system via fatty pads on the bottom of the foot, in amputees, as shown in a schematic diagram 20 in FIG. 1, loads are transferred from the ground to a prosthetic socket 26 via a prosthetic foot 22 and a rigid aluminum tube called a pylon 24. The prosthetic socket then transfers the loads to the soft tissues of a stump 28, or residual limb. When performing turning maneuvers, amputees experience increased transverse plane torques. These increased transverse plane torques result in increased shear stresses that are believed to be associated with pain, and the formation of epidermoid cysts and ulcers, which can require several visits to a physician to manage. Thus, new technology is needed to improve comfort and prevent injury related to transverse plane torques during amputee turning gait.
The intact human ankle can serve as inspiration for a prosthetic device to reduce torsional loads while turning, since the actual ankle permits limited motion in the transverse plane. Previous research has investigated transverse plane ankle behavior and found that it behaves as a passive system with variable stiffness, both throughout the gait cycle and between straight and different turning steps. Despite this behavior, previous attempts to reduce transverse plane loading have focused on technologies with fixed stiffness.
To protect the soft tissues from the effects of torsional loads, which can impart painful and potentially injurious shear stresses upon residual limb soft tissues in lower limb amputees, prosthetic manufacturers have developed transverse rotation adapters (TRAs) that are essentially torsional springs mounted in the pylon of the prosthesis, which permit the prosthetic socket to rotate relative to the prosthetic foot, relieving some of the load acting on the residual limb. A variety of stiffness values are available for TRAs, allowing prosthetists to choose linear and nonlinear options and even allowing different values to be chosen for external and internal rotation. But once installed in the TRA, the stiffness does not vary as it does in the human ankle While it would be advantageous to be able to adjust the stiffness of the torsional spring to suit different activities, once installed, the stiffness of conventional TRAs cannot be easily adjusted. Any adjustment requires removal of the prosthesis and either replacement of the torsion spring or adjustment of the spring force provided by the torsion spring.
Another passive strategy that has been employed to reduce transverse loading is embodied in the Rotasafe™ device. This device is essentially a slip-clutch designed to prevent over-rotation of osseointegrated implants. Slip-clutches use static friction to maintain torsional rigidity until a certain torque is reached, at which point, the slip-clutch allows rotation, which, in the case of the Rotosafe™, acts to prevent damage to the bone-implant interface. Effectively, this device enables a binary variation selection between stiffness values (mainly, one very stiff and one soft), but cannot replicate the multitude and range of elastic behaviors exhibited by the human ankle. Furthermore, while the device can save an implant from excessive torques, the slipping rotations might induce falls that can cause other injuries.
In addition to preserving residual limb health, another important challenge for lower limb prosthetic design is to improve the metabolic cost of walking for amputees. Walking with a prosthesis requires much more metabolic energy to walk than is expended by a person with intact limbs. Indeed, dysvascular transfemoral amputees require more than twice as much oxygen to walk a meter than intact individuals. Other amputee levels and etiology also require considerably more oxygen to walk than intact individuals.
The cause of the elevated metabolic cost associated with amputee gait is largely unknown, and most of the research in the area has focused on how different prosthetic components affect metabolic cost. Prosthetic feet are some of the more popular components studied. With the advent of flexible energy storage and release feet, a number of researchers have investigated whether these feet can reduce the metabolic cost of walking Unfortunately, only three of the studies (of nine total) were able to detect differences in metabolic cost. Furthermore, one of those studies detected differences only for higher walking speeds, and the differences in the other studies were so small as to lack clinical significance, despite their statistical significance. Thus, it appears that energy storage and release feet have had limited success in reducing the metabolic cost of walking for amputees.
With the advent of microprocessor-controlled prosthetic knees, researchers again raised the question of whether prosthetic technology could reduce the metabolic cost of walking and again had mixed results. One group of researchers compared the C-Leg to the Mauch SNS prosthetic knee with eighteen transfemoral amputees and was unable to detect a difference in metabolic cost. Others compared the C-Leg, Rheo, and Mauch SNS knees and found that amputees had 3% and 5% lower metabolic rates with the C-Leg and Rheo knees, respectively, than with the Mauch SNS. Still another research group found that the C-Leg reduced the metabolic cost by 6% compared to a mechanical knee. Accordingly, while some studies were able to detect a metabolic benefit of using microprocessor-controlled prosthetic knees, these benefits were small compared to the enormous metabolic losses associated with transfemoral amputee gait. Furthermore, it should be noted that the two studies that detected metabolic benefits were funded by manufacturers of microprocessor-controlled knees, while the study that did not detect a benefit was funded by the U.S. government.
While innovations in energy storage and release feet and microprocessor-controlled knees have been unable to meaningfully decrease the metabolic cost of walking for lower limb amputees, recent research with inverted pendulum models of gait may hint at more fruitful interventions. Historically, walking has been believed to employ six kinematic features of gait to reduce the vertical displacement of the body center of mass (COM) in order to minimize metabolic cost. The inverted pendulum theory of gait proposes instead that the stance limb behaves like an inverted pendulum and that there are metabolic benefits associated with exploiting this natural dynamic behavior. With the inverted pendulum theory, step-to-step transitions are major sources of metabolic cost. More specifically, in order to redirect the COM along another pendular arc at the end of a step, the leading and trailing limbs perform negative and positive work simultaneously which exerts a metabolic cost. It has been found that transtibial amputees have difficulty generating positive work when the prosthetic leg trails, suggesting that a powered prosthetic ankle might decrease the step to step transitions and, consequently, the metabolic cost. Indeed, preliminary results with a powered sagittal ankle system have been able to reduce metabolic cost by an average of 14% with three subjects. In addition to the sagittal plane, considerable work must be performed to redirect the COM in the frontal plane, as well, suggesting that more metabolic gains can be achieved with active technology to propel the COM in this plane.
Accordingly, a new generation of technology is needed that enables the transverse loading to be varied across a wide range of stiffness values and/or torque values. It would also be desirable to employ an active approach to control the effective stiffness of rotation in a prosthesis, to be more responsive to loading changes. It would also be desirable to enable the stiffness of the loading to be readily varied with a control to enable an amputee to more effectively engage in various activities that benefit from the application of different levels of torsional stiffness. As a further benefit, the use of such a prosthesis should substantially reduce the metabolic cost to the subject by providing a gait that more closely replicates that of an intact individual.